In general, thin-film fabricating processes are ordinarily used in order to form thin-films such as a polysilicon film, an oxide film, and a nitride film on a semiconductor substrate in a semiconductor device by using the device for a thermal treatment process and a film forming process and a chemical vapor deposition (CVD) device, for example. After completion of the processes these thin-films formed on the semiconductor substrate become components of the semiconductor device.
In general, the degree of cleanliness of the surfaces of a semiconductor substrate before forming of thin-films thereon greatly affects features and reliability of the semiconductor device.
In order to eliminate minor effects of the poor degree of cleanliness of the semiconductor substrate, many kinds of wafer and semiconductor substrate cleaning processes are executed before a thin film forming process or among thin-film forming processes.
However, in a conventional device for a thermal treatment process and a thin-film forming process, oxidative reaction is often happened on a semiconductor substrate when the semiconductor substrate is inserted to and removed from a chamber in a device for a thermal treatment process and a thin-film forming process or a chemical vapor deposition (CVD) device because the chamber is at a very high temperature.
The oxide film formed on a semiconductor substrate reduces the characteristic of the semiconductor device and reliability thereof. This is a problem.
FIG. 1 shows a configuration of a conventional device for thermal treatment and a thin-film fabricating process. As shown in FIG. 1, a heater 103 is adhered on a chamber 101 in the conventional device to heat the chamber 101 as a thermal treatment. In addition, in order to keep a constant temperature in the chamber, an insulator 105 as a temperature keeping material is adhered on the heater 103 to cover the chamber 101.
As clearing shown in FIG. 1, there is no cooling device to cool the heater 103 in a conventional device for a thermal treatment and a thin-film forming process.
Accordingly, the conventional device for a thermal treatment process and a thin-film forming process is only provided with means for lowering the temperature within the film-forming process chamber 101 consisting of a cooling process relying on natural heat dissipation, for preventing an oxidation reaction caused by heating.
Specifically, the temperature of the heater 103 and the temperature in the film-forming process chamber 101 are almost equivalent because of the above-described structure of the conventional film-forming chamber.
Even when it is desired to cool the inside of the film-forming chamber 101, there is no function provided to cool the heater 103 itself.
It is therefore necessary to rely on cutting the power to the heater 103 and allowing natural dissipation of the heat. Considerable time is therefore required to attain the desired lowering of the temperature in the film-forming chamber 101.
To explain the above-mentioned drawback of the conventional deice used for a thermal treatment process and a thin-film forming process in more detail, the temperature of the heater 103 after a thin-film is formed on the semiconductor substrate conventionally decreases at a cooling rate of about 2.degree. C. per minute to reach a temperature at which a natural oxide film will not grow, so that the next thin-film can be formed. Therefore, for example, about 100 minutes elapses in cooling 200.degree. C. from 600.degree. C. to 400.degree. C.
This cooling time is equivalent to or greater than the time required in the original processing, for example, lowering the pressure after setting a wafer, forming a film, returning to normal pressure, and resetting a wafer. When this amount of time is used simply to lower the temperature, there is an increase in the amount of wasted time not directly related to production, resulting in a major drop in productivity.
For this reason, it frequently happens that the wafer is normally set in the film-forming chamber 101 at the processing temperature, without decreasing the temperature in the film-forming chamber 101. Therefore, even when the oxide film naturally formed on the surface of the silicon is removed prior to processing, air penetrating the surface of the wafer as a result of air entering the chamber during the insertion of the wafer promotes an oxidation reaction as a result of the high temperature inside the film-forming chamber.
A thin oxide film therefore grows before a film of polysilicon or the like is formed. Under such conditions, when an emitter section of a transistor is formed from a polysilicon film, this natural oxide film acts as a barrier, inhibiting the diffusion of impurities throughout the silicon, so that the device has high emitter resistance.
However, recent progress has been made in the miniaturization of such devices, and, when the size of the emitter region is reduced, it is necessary to hold the emitter resistance to a low level. This type of phenomenon by which an oxide film is formed is therefore inconvenient.
In addition, during maintenance of the device for the thermal treatment process and the film forming process, it takes about half a day to cool the device to room temperature in order to wash tubes in the device. Processing must therefore be halted during this period, and this hinders productivity.